In order to enhance the efficiency of optical coupling of a light beam from a light source such as a laser diode device with an optical fiber, there has been used a lensed optical fiber in which a lens is formed on the end face of an optical fiber. For example, as shown in FIG. 4, a lens 2 formed on the end face of an optical fiber 1 has an external form consisting of a hyperboloidal portion 2a and a spherical portion 2b inscribed in the hyperboloidal portion 2a. The optical fiber 1 having the lens 2 of this form has a high efficiency of optical coupling with a light source. For a laser diode having a generating wavelength of 0.98 .mu.m at the center, the maximum optical coupling efficiency can be obtained when a radius R of the spherical portion 2b inscribed in the hyperboloidal portion 2a is generally 1.5 to 5 .mu.m (see U.S. Pat. No. 5,256,851).
As shown in FIG. 5, there also has been proposed an optical fiber having a lens 12 of a wedge-shaped external form having two-stage tapered portions 12a and 12b with different angles of .theta.1 and .theta.2, respectively (see U.S. Pat. No. 5,455,879). An optical fiber 11 having a lens 12 of the form shown in FIG. 5 also exhibits a high optical coupling efficiency.
However, the aforementioned lensed optical fibers have the following problems:
1) For the lens 2 of the first form shown in FIG. 4, the hyperboloidal portion 2a is formed by being polished using a flat plate polishing machine while the angle between the optical fiber and the polishing machine is adjusted. Therefore, for the lens 2 of the first form, it is difficult to fabricate the hyperboloidal portion 2a with high reproducibility, so that the yield decreases. Also, the optical coupling efficiency varies in a sensitive manner depending on the diameter of the spherical portion 2b, which also results in a decrease in yield.
For the lens 2 of the first form, morever, it is difficult to accurately determine the boundary between the hyperboloidal portion 2a and the spherical portion 2b when the radius of the spherical portion 2b inscribed in the hyperboloidal portion 2a is measured. Therefore, the lens 2 of this form presents a problem in that inspection of the external form takes much time.
2) The lens 12 of the second form shown in FIG. 5 is polished while the angle between the optical fiber 11 and the polishing machine is kept constant. Therefore, the lens 12 of the second form can be fabricated more easily than the lens 2 of the first form. However, for the lens 12 of the second form, because two-stage tapered portions 12a and 12b with different angles of .theta.1 and .theta.2 are formed, the fabrication takes much time, and the reproducibility of form and increase in yield are limited.
Accordingly, an object of the present invention is to provide a lensed optical fiber which overcomes the aforementioned drawbacks of the conventional lensed optical fibers and, more specifically, to provide a lensed optical fiber which has a high efficiency of optical coupling with a light source, and which can be fabricated with high fabrication accuracy and high yield.